Hearing impairment is most commonly experienced as loss of sensitivity to weak sounds, while intense sounds can be as loud and uncomfortable as in normal hearing. State-of-the-art hearing aids treat this phenomenon of "loudness recruitment" with sound amplification that automatically decreases with sound amplitude. This compresses the range of normally experienced sound amplitudes to the smaller range required by the impaired ear. The best engineering approach to compression is uncertain. Rapid compression amplifiers protect the ear from uncomfortable changes in loudness, but distort the sound waveform. Slowly adapting compression avoids the distortion, but allows some discomfort. Basic research on cochlear responses to sounds has revealed biological mechanisms for both rapid and slow compression that are normally used in hearing. In research sponsored by the National Institutes of Health, the principal investigator has developed models of nonlinear cochlear sound processing, which suggest optimum compression strategies that maximize protection against uncomfortable loudness while minimizing distortion. Algorithms for improved multichannel hearing aids based on these models have been developed and systematically simulated. The goal of Phase I is to implement the algorithms as a prototype master hearing aid that can be used for clinical testing and development of commercial designs. PROPOSED COMMERCIAL APPLICATION: The master hearing aid has commercial application as a clinical and industrial research instrument for the fitting and design of hearing aids. Clinical research in Phase II will quantify the benefits of the novel designs and guide their commercialization.